Say you've got a modest-sized church with an annual budget of $200,000. A new member joins who makes $1 million a year and he starts tithing $100,000. What should you do? There are a few options.
1. Raise the budget to $300,000. This is probably what most churches would do, but it doesn't seem like a wise decision to me. The church would be dependent on one person for a third of its income, and individual jobs can be very volatile. The giver could die, retire, move, change churches, go to jail, get divorced, get fired, or even try to dictate church policy using his money. If the money he was giving is spent on normal budget items and recurring expenses, the loss of revenue could easily lead to foreclosed mortgages or laid-off staff members.
2. Invest it. I like this idea. The church can then add the interest to its regular budget without worry about the giver losing his job. However, from a spiritual standpoint it may not be the best solution. Churches are supposed to operate off giving, not business investments, and the government might not like a non-profit making money this way.
3. Spend it on non-recurring expenses. Rather than adding the new money to the budget, it makes more sense to spend most of it on non-recurring items. For example, rather than hiring a staff member who would need to be laid-off, the church could buy property (for cash, not a mortgage!). The church could also do construction or other improvements. The idea is to avoid financial committments that would be endangered if the big giver stops giving.
In general, I think it would be wise for a church to prevent any one giver from supporing more than, say, 10% of its recurring expenses and regular budget items. In the example above, the church could add $20,000 to its budget for a total of $220,000, and then spend the other $80,000 each year on non-recurring items. The solution may seem a bit awkward, but I think it's safer than becoming overly dependent on any individual.